Tanks donated by the UK to Ukraine have been deployed as part of Kyiv's ongoing incursion into Russia, British media reported on Thursday.
It is believed to be the first time the Challenger 2 tanks have been used in active combat on Russian territory, Sky News and other UK outlets said.
They did not specify how many of the tanks were thought to have been deployed by Ukraine in its surprise incursion into Russia's Kursk region, which began on August 6.
The UK Ministry of Defence declined to comment on what specific weaponry Ukraine was using, but noted "operations inside Russia" had not been precluded when the tanks and other arms were donated to Kyiv.
"There has been no change in UK government policy," a ministry spokesman said.
"Under Article 51 of the UN Charter Ukraine has a clear right of self-defence against Russia's illegal attacks, that does not preclude operations inside Russia.
"We make clear during the gifting process that equipment is to be used in line with international law."
Ukraine sent troops and armoured vehicles into the Kursk region nine days ago, claiming the capture of several dozen villages in the most serious cross-border attack on Russian soil since World War II.
Moscow on Thursday said it would send more troops to defend the neighbouring border region of Belgorod amid the ongoing Ukrainian assault.
British equipment supplied to Kyiv since Russia invaded more than two years ago includes Storm Shadow cruise missiles and a squadron of 14 Challenger 2 tanks deployed early last year.
The Times reported Wednesday that former UK defence minister Ben Wallace had disclosed that during his tenure he had given permission for Ukraine to hit targets inside Russia using "whatever weapons [were] supplied to it".
However, he noted that restrictions had been placed on the use of the long-range Storm Shadow missiles.
The newspaper added that he believed it was "perfectly legitimate" for Ukraine to use other UK-gifted weapons within Russia if the attack was designed to target logistics and infrastructure supporting Moscow's invasion.
Britain's current Defence Secretary John Healey, who took up the post in early July after Labour swept to power, has said the UK remains "united for Ukraine".
"As the new defence Secretary, I will ensure that we reinvigorate Britain's support by stepping up supplies of vital military aid," he said within days of being appointed.
